Economies worldwide nearly ground to a halt over the 15 months of the coronavirus pandemic, leading to a startling drop in global greenhouse gas emissions.

But the idle airplanes, boarded-up stores and quiet highways barely made a dent in the steady accumulation of carbon dioxide in the atmosphere, which scientists from the Scripps Institution of Oceanography and the National Oceanic and Atmospheric Administration said Monday had reached the highest levels since accurate measurements began 63 years ago.

The new figures serve as a sober reminder that even as President Joe Biden and other world leaders make unprecedented promises about curtailing greenhouse gas emissions, turning the tide of climate change will take even more massive efforts over a much longer period of time.

The report of a climb in atmospheric carbon dioxide was also published on the eve of a meeting of the Group of Seven industrialized countries, where climate change is expected to be at center stage. The G-7 meeting is intended to prod major emitting countries toward more ambitious actions ahead of a major international climate conference in Glasgow in November.

“Fossil fuel burning is really at the heart of this. If we don’t tackle fossil fuel burning, the problem is not going to go away,” Ralph Keeling, a geochemist at Scripps, said in an interview, adding that the world ultimately will have to make emissions cuts that are “much larger and sustained” than anything that happened during the pandemic.

Scientists from Scripps and the NOAA said on Monday that levels of atmospheric carbon dioxide peaked in May, reaching a monthly average of nearly 419 parts per million.

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That represents an increase from the May 2020 mean of 417 parts per million, and it marks the highest level since measurements began 63 years ago at the NOAA observatory in Mauna Loa, Hawaii. Twice in 2021, daily levels recorded at the observatory have exceeded 420 parts per million, researchers said.

“While 2020 saw a historic drop in emissions, the fact that at certain points more than half the world’s population was under lockdown, and emissions ONLY fell 6 percent, should be a sobering reminder of how staggeringly hard it will be to get to net zero and how much more work we have to do,” Jason Bordoff, founding director of Columbia University’s global energy center, said in an email.

“Shutting down economic activity is not a viable or desirable way to reduce emissions, and as economies open back up, it’s not surprising emissions are rising because we still have not put in place the changes needed for the overall system of how we produce and consume energy.”

The increasing CO2 concentration alone was not particularly surprising to scientists, who have watched the figure rise steadily over time. What was telling was the fact that the drop in emissions during the pandemic did little to slow down the increase.

“It’s significant in that it shows we are still fully on the wrong track,” Pieter Tans, a senior scientist with NOAA’s Global Monitoring Laboratory, said in an interview. “The rate of increase has been the highest in the past decade, and we’re still on it.”

Tans noted that humans continue to add about 40 billion metric tons of carbon dioxide pollution to the atmosphere each year, and that avoiding catastrophic changes to the climate will require reducing that number to zero as quickly as possible.

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“The emissions of CO2 continue to be incredibly high,” said Corinne Le Quéré, research professor of climate change science at the University of East Anglia in Britain. “The concentrations of CO2 in the atmosphere will stop rising when the emissions approach zero.”

Doing that translates into daunting targets. In November 2019, a United Nations Environment Program report warned that unless global greenhouse gas emissions fell by 7.6% each year between 2020 and 2030, the world would miss the opportunity to limit warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit), above preindustrial levels — a key goal of the Paris agreement.

Last year, a report by the same group said that to meet that goal, countries would need to increase their current emissions-cutting pledges fivefold — an aspiration that would require rapid and profound changes in how societies travel, produce electricity and eat.

Carbon dioxide, a key greenhouse gas, traps heat from the planet’s surface that would otherwise escape into space. Much of the carbon dioxide breaks down after about 100 years, but the current global rate of emissions is enough to offset that rate and further increase the atmospheric concentration of the gas, causing the planet to warm steadily.

The highest monthly mean levels of carbon dioxide typically occur each May, just before plants in the Northern Hemisphere start to remove large amounts of CO2 from the atmosphere during the growing season. In the northern fall, winter and early spring, plants and soil give off CO2, causing levels to rise.

Even as international borders closed and global economic activity took a massive hit throughout much of 2020, researchers have found that human-caused emissions rebounded fairly quickly after decreasing sharply early in the pandemic.

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In 2020, primary energy demand decreased nearly 4%, and global energy-related carbon dioxide emissions fell by 5.8%, according to the International Energy Agency — the largest annual percentage decline since World War II.

In absolute terms, the decline in emissions is “without precedent in human history,” the IEA said. “Broadly speaking, this is the equivalent of removing all of the European Union’s emissions from the global total.” The agency said that demand for fossil fuels was hardest hit in 2020 — especially oil, which plunged 8.6%, and coal, which dropped by 4%.

But in the broader sense, the pandemic could prove to be little more than a blip in the world’s efforts to combat climate change.

Energy-related carbon dioxide emissions during 2020 dropped to about the same level of global emissions that prevailed in 2012 — not nearly low enough to change the world’s current trajectory. That reality offers the latest evidence of the stubbornness of human-related emissions and the difficulty the world faces in making the kind of far-reaching, long-lasting cuts necessary to slow Earth’s warming and avoid the most catastrophic effects of climate change.

Already, the IEA has said it expects global carbon emissions to surge this year as parts of the world rebound from the coronavirus pandemic. The group projected in April that emissions are on track to reach the second-largest annual rise on record.

Global energy demand is already set to surpass 2019 levels, alongside continued growth in alternative energies, the Paris-based organization found.

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As levels of atmospheric carbon dioxide continue to surge, leaders around the world face mounting pressure to commit to more aggressive, more urgent plans to reduce greenhouse gas pollution. Some countries have begun to outline more ambitious targets ahead of a key U.N. climate conference in the fall. Among them is the United States, which under President Joe Biden has vowed to cut its overall emissions in half by the end of the decade.

Still, analyses by the United Nations and other organizations have found that a grim gap remains between the world’s current path and the significant shifts needed keep Earth’s warming to “well below” 2 degrees Celsius (3.6 Fahrenheit) above preindustrial levels — a central goal of the Paris agreement. In short, the existing promises aren’t enough, and most countries have not lived up to the promises they have made.

Keeling said he is optimistic that major changes lie ahead as renewable energy and other technologies take root and multiply. But they won’t happen overnight. “I do expect we will see significant changes in the years ahead. The political will has shifted,” he said. “What we need to do is see a sustained move toward moving away from fossil fuels.”

Tans also holds out hope that the world will be able to put itself on a better path. The science of how to do that exists, he said, but what remains unclear is whether societies can muster the kind of action that has yet to materialize.

“The goals so far are themselves insufficient, even after having been beefed up,” he said. “We’re running out of time. The longer we wait, the harder it gets.”